1. Field of the Invention
The present invention is directed to improved lubricants for the compaction and sintering of powdered materials, and specifically to a lubricant including vinyl halides and more specifically, to a lubricant including polytetrafluoroethylene and other vinyl halides for the compaction and sintering of ferrous alloys, non-ferrous metals and non-metals. The improved lubricant results in compacted materials having improved powder flow, improved powder packing, higher green densities, reduced die wear, reduced die scoring and reduced die seizure during ejection with less admixed lubricant and the ability to increase lubricant concentrations without negatively affecting green strength. The improved properties during sintering include less pollutants, a clean burn resulting in no soot remaining after delubing, reduced dimensional change during sintering, and a cleaner powder surface as the vinyl halide acts as a "getter" and tends to clean the powder surface before sintering occurs and the existence of smaller pores in the compacted material after burnout of the lubricant. The improved post-sintering properties include higher sintered densities, greater sintered tensile strength, the ability to increase lubricant content without negatively affecting sintered tensile strength and increased hardness of the compacted material.
2. Background Information
In the typical compaction process, a material in the form of a powder, which is typically either a ferrous alloy, a non-ferrous alloy or a non-metal, is compacted in a press in a die to form a compact of a desired configuration.
The density of the compacted material immediately after compaction is referred to as its "green" density. Upon removal from the die, the compacted material is sintered. The density after sintering is referred to as the "sintered" density.
Recently, automotive and other industry users of parts and components comprised of compacted/sintered materials have looked for increased green densities and strengths over an increasingly wider compaction range. Increased green density results in improved properties after sintering, including increased hardness and tensile strength of the compacted/sintered component. Unfortunately, the higher density pressing of compacted materials, including ferrous alloys, non-ferrous metals and non-metals, increases the rate of die wear.
Lubricants, when added to the powdered material to be compacted, decrease die wear and improve the compaction process. The lubricants can be introduced into the material to be compacted as a powder or they can be sprayed as a coating into the die cavity and onto the punches.
The two most common lubricants are zinc stearate and ethylene-bistearamide (hereinafter "EBS"). Each lubricant has advantages and disadvantages over the other. EBS results in lower dimensional swelling of the component after compaction, however it results in components exhibiting lower green and sinter densities with respect to zinc stearate. On the other hand, zinc stearate is environmentally unfriendly and leaves residues after burnout, while EBS can be easily removed from the compacted material during delubing. Further, removal of EBS from a compacted material requires far less hydrogen and lower temperatures than required to remove the same amount of zinc stearate. Therefore, an improved lubricant would include the best properties of both zinc stearate and EBS without their attendant disadvantages.
Efforts have been made in the art to improve green densities and other green physical properties and, in turn, sintered densities and sintered physical properties by focusing on improving the lubricant itself. Lubricant characteristics, such as particle size, thermal stability, coefficient of friction and true densities, are being studied to promote the highest green densities possible. Nonetheless, a need remains in the art for a lubricant which can improve green densities, other green physical properties, sintered densities and sintered physical properties, which is environmentally acceptable, is clean burning and can be easily removed at lower temperatures while providing those desired higher green densities and strengths over the entire compaction range.